As you will recall, our food has left the stomach and arrived at the duodenum, the first section of the small intestine. There it has encountered trypsin, chymotrypsin and carboxypolpeptidase for protein and peptide digestion, pancreatic amylase to digest starches and pancreatic lipase for fat digestion. Though some pancreatic enzymes are secreted in their active form, optimal activity of these pancreatic enzymes requires the presence of bile.

One of the many functions of the liver is to produce and secrete bile. Bile is secreted continuously by liver cells, and is either emptied directly into the duodenum or is diverted and stored in the gallbladder.

Liver bile is composed mostly of bile salts (about one half the total volume) as well as: bilirubin, cholesterol, lecithin, and electrolytes found in plasma. The gallbladder not only stores liver bile, but concentrates it up to 15 times the original volume by reabsorbing a large portion of the electrolytes and water. This creates a highly condensed combination of bile salts, lecithin, cholesterol, bilirubin and calcium ions.

Though bile contains no digestive enzymes itself, it provides three important digestive functions:

First, it has a detergent or emulsification action on fat particles. A major function of lecithin and bile salts is to reduce large fat globules. It does this by lowering the surface tension of the globules, allowing them to brake apart during peristalsis.   Pancreatic lipase enzymes are water-soluble and therefore can only interact with fat on its surface. Each time the diameter of a fat globule splits in two, the total surface area doubles. The emulsification process of bile increases the surface area of fat up to 1000 times, allowing pancreatic lipase greater accessibility to fat molecules. Essentially, all fat digestion occurs in the small intestine.

Second, bile aids in the transport of free fatty acids, monoglycerides, cholesterol and other lipids to the mucosal wall of the small intestine. It does this by forming Micelles –highly absorbable, electrically charged lipid complexes. These micelle complexes are drawn to the intestinal mucosal lining where they are absorbed into the body.   Without bile salts 40% of these nutrients would be excreted from the body.

Third, bile serves as a means to excrete several important waste products from the blood. Bilirubin, an end product of hemoglobin destruction, and excess cholesterol are both excreted in combination with bile into the digestive tract, and subsequently removed from the body.

The presence of fatty foods entering the duodenum, usually about 30 minutes after a meal, stimulates the gallbladder to release bile.  If the meal does not contain fat,  the gallbladder releases less, but when adequate quantities of fat are present the gallbladder empties completely.

After fat digestion is complete, about 94% of the bile salts will be reabsorbed to be used again by the liver as bile for fat digestion.

Next we will examine digestion in the small intestine.